Score One for the Sociable Ape

Chimpanzees and bonobos are our closest primate relatives, but the two apes have very different personalities. While primatologists have often noted nasty, competitive behavior among power-hungry chimps, bonobos have a reputation as free-loving peaceniks. Now, a behavioral study that directly compares the two apes suggests that the bonobos' more cordial nature enables them to cooperate more successfully than chimps in some situations.

Most researchers believe that humans shared a common ancestor with chimpanzees and bonobos between 5 million and 7 million years ago (for a different take, see ScienceNOW, 27 February). Both of these apes may have something to tell us about the evolution of human behavior, yet most research has focused on chimps, in large part because bonobos are endangered--perhaps as few as 10,000 remain. In the new study, researchers worked with bonobos at a sanctuary in the Democratic Republic of Congo and with chimps at a Ugandan sanctuary.

The different natures of the two apes became clear when the researchers, led by Brian Hare, a biological anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, presented pairs of bonobos and pairs of chimps with plates of fruit. Bonobo pairs reacted by playing with each other and even rubbing genitals--a frequent stress-reliever in bonobo society. They also shared the bounty more often than not. Chimps, on the other hand, generally avoided their partner and shared food less than half of the time.

Next, the researchers tested both apes on a cooperation task they'd used previously with chimps, putting pairs of animals in an enclosure and placing a fruit-laden platform just outside. The only way to bring the food within reach was to simultaneously pull two ropes connected to the ends of the platform, but the ropes were too far apart for one animal to reach on its own (ScienceNOW, 2 March 2006). Both chimps and bonobos teamed up with their cagemate to pull the ropes when the fruit was cut up into easily sharable pieces, the researchers found. But when the food was cut into big chunks, bonobos cooperated to haul in the fruit more often than chimps did. And when the chimps did cooperate, they almost always adopted a winner-take-all mentality, with one animal hogging the entire bounty, the researchers report online today in Current Biology. Because bonobos are more tolerant of each other and more willing to share, they're able to cooperate more effectively than chimps in some situations, the researchers conclude.

The findings "open a bit of a door on the bonobo mind that we didn't have before," says Frans de Waal, a primatologist at Emory University in Atlanta, Georgia. Although field studies have found little evidence of cooperative behavior in wild bonobos, it may simply be that their lush forest habitat provides enough easily accessible food that teamwork isn't necessary, de Waal says. We often look to chimps for clues to human behavior, he notes, but "this study shows another side to our primate ancestry."